Upregulation of N-type calcium channels in the soma of uninjured dorsal root ganglion neurons contributes to neuropathic pain by increasing neuronal excitability following peripheral nerve injury

N-type voltage-gated calcium (Cav2.2) channels are expressed in the central terminals of dorsal root ganglion (DRG) neurons, and are critical for neurotransmitter release. Cav2.2 channels are also expressed in the soma of DRG neurons, where their function remains largely unknown. Here, we showed that Cav2.2 was upregulated in the soma of uninjured L4 DRG neurons, but downregulated in those of injured L5 DRG neurons following L5 spinal nerve ligation (L5-SNL). Local application of specific Cav2.2 blockers (omega-conotoxin GVIA, 1-100 mu M or ZC88, 10-1000 mu M) onto L4 and 6 DRGs on the operated side, but not the contralateral side, dose-dependently reversed mechanical allodynia induced by L5-SNL. Patch clamp recordings revealed that both w-conotoxin GVIA (1 mu M) and ZC88 (10 mu M) depressed hyperexcitability in L4 but not in L5 DRG neurons of L5-SNL rats. Consistent with this, knockdown of Cav2.2 in L4 DRG neurons with AAV-Cav2.2 shRNA substantially prevented L5-SNL-induced mechanical allodynia and hyperexcitability of L4 DRG neurons. Furthermore, in L5-SNL rats, interleukin-1 beta (IL-1 beta) and IL-10 were upregulated in L4 DRGs and L5 DRGs, respectively. Intrathecal injection of IL-1 beta induced mechanical allodynia and Cav2.2 upregulation in bilateral L4-6 DRGs of naive rats, whereas injection of IL-10 substantially prevented mechanical allodynia and Cav2.2 upregulation in L4 DRGs in L5-SNL rats. Finally, in cultured DRG neurons, Cav2.2 was dose-dependently upregulated by IL-1 beta and downregulated by IL-10. These data indicate that the upregulation of Cav2.2 in uninjured DRG neurons via IL-1 beta over-production contributes to neuropathic pain by increasing neuronal excitability following peripheral nerve injury.